Coatings are often applied to equipment subjected to harsh environments or operating conditions in efforts to extend the useful lifetime of the equipment. Various coating identities and constructions are available depending on the mode of failure to be inhibited. For example, wear resistant, erosion resistant and corrosion resistant coatings have been developed for metallic substrates.
A significant problem encountered in coating applications is premature failure or degradation of the coating. Coatings of metallic substrates can fail according to a variety of mechanisms, including delamination and cracking/fracture. In some cases, a coated metal substrate is subjected to thermal cycling that can impair the bonding of the coating to the substrate. In some applications, for example, the metal substrate is subjected to a post-coat heat treatment such as martermpering or normalizing in order to improve the mechanical properties of the substrate, wherein the post-coat heat treatment fractures the coating. Moreover, in some cases, cracks in the coating can propagate into the substrate leading to additional problems. FIG. 1 illustrates fracture of an abrasion resistant prior art coating bonded to a metal substrate, the fracture resulting from coating processes and/or heat treatment of the substrate. As illustrated in FIG. 1, the crack traversed the abrasion resistant coating.
Attempts have been made to provide coating architectures that are resistant to premature failure resulting from heat treatment, cladding processes and other environmental factors. In U.S. Pat. No. 5,352,526, for example, a composite coating is provided having a soft metallic layer under a hardface coating, the soft metallic layer having a crack arrest functionality. The composite coating of U.S. Pat. No. 5,352,526 is formed by stacking preform layers on the metal substrate surface and heating the preform layers in a single step to provide the composite coating. A preform comprising particles of the soft metallic underlayer is applied to the substrate followed by application of a preform of refractory particles, such as tungsten carbide. A braze filler preform is applied to the refractory particle preform and all three preforms are heated simultaneously to provide the coating. The braze filler material top layer is infused by capillary action into both the porous refractory particle layer and the porous soft metal particle layer yielding an essentially void free coating. While demonstrating sufficient crack arrest properties, the soft layer permitted crack propagation beyond the interface with the hard particle layer, thereby compromising the mechanical and corrosion resistant properties of the soft layer.